This invention relates to a process for making aluminum alloy can stock, viz., aluminum alloy sheet for forming one-piece drawn and ironed can bodies, and to the product of such process.
Present-day metal cans as used for beverages such as soft drinks, beer and the like are commonly constituted of a seamless one-piece body (which includes the bottom end and cylindrical side wall of the can) and a top end bearing a ring or other opening device. The body is produced from a blank of cold-rolled aluminum alloy sheet (having a gauge, for example, of about 0.014 inch) by a now-conventional forming technique known as drawing and ironing, which involves drawing the blank into a cup and then passing it through a succession of dies to achieve the desired elongated cylindrical body configuration, with a side wall of reduced thickness relative to the bottom end. The top end is separately produced from another sheet aluminum alloy blank, by different but also conventional forming operations, and is secured around its circumference to the top edge of the side wall of the body to provide a complete can.
The severity of the forming procedure employed in producing a drawn-and-ironed can body as described above, and in particular the reduction in thickness of the can side wall (which must nevertheless be able to withstand the internal and external forces exerted on it in use), as well as the fact that the formed can is usually lacquered in an operation necessitating a strength-reducing exposure to heat, require a special combination of strength, formability, and tool wear properties in the alloy sheet from which the can body is made. Significant among these properties are ultimate tensile strength, yield strength, elongation, and earing. Attainment of the requisite combination of properties is dependent on alloy composition and on the processing conditions used to produce the sheet.
Heretofore, a conventional sheet for can body blanks has been constituted of the alloy having the Aluminum Association (AA) designation 3004, and has been produced from conventionally direct-chill-cast ingot up to 24 inches thick by scalping and homogenizing the ingot, and successively hot rolling and cold rolling to the desired final gauge; often an anneal treatment is used between the hot and cold rolling operations, with the annealing gauge so selected that the amount of cold reduction to final gauge after annealing is about 85%, thereby to provide can body blanks in H19 (extra hard) temper. Copending U.S. patent application Ser. No. 211,644 (now U.S. Pat. No. 4,318,755, issued Mar. 9, 1982), filed Dec. 1, 1980, by Paul W. Jeffrey (one of the applicants herein) and John C. Blade for Aluminum Alloy Can Stock and assigned to the same assignee as the present application, describes can body stock comprising aluminum alloy sheet at an intermediate temper and directly formable by drawing and ironing into a one-piece can body, containing 0.45-0.8% Mn and 1.5-2.2% Mg, with the following properties: ultimate tensile strength, at least about 38 thousand pounds/in..sup.2 (k.p.s.i.); yield strength, at least about 35 k.p.s.i.; elongation, at least about 1%; earing, not more than about 4%. It will be understood that all composition percentages above and elsewhere herein are expressed as percentages by weight.
It would be desirable to utilize, e.g. in the manufacture of can body stock, so-called continuous strip casting techniques in place of conventional direct-chill casting of relatively thick ingots. Continuous strip casting is performed by supplying molten metal to a cavity defined between chilled, moving casting surfaces such as substantially parallel, extended planar runs of a pair of chilled endless metal belts, thereby to produce a thin (typically less than one inch thick) continuous cast strip. Belt-casting apparatus for such casting of strip is described, for example, in U.S. Pat. Nos. 4,061,177 and 4,061,178, the disclosures of which are incorporated herein by this reference. Advantages of continuous strip casting (as compared with direct chill casting of thick ingots) for production of sheet aluminum alloy products include enhanced efficiency and economy, especially in that the thinness of the as-cast strip significantly lessens the extent to which the cast body must be reduced by rolling to a desired sheet gauge. Heretofore, however, it has not been feasible to produce sheet for one-piece can bodies from belt-cast strip because AA 3004 alloy rolled from such strip to provide sheet of can body stock gauge at H19 temper does not possess satisfactory properties for commercial drawing and ironing into one-piece can bodies, owing to differences in work-hardening rate, earing, and required annealing temperature between strip-cast and direct chill-cast AA 3004 products.
U.S. Pat. No. 4,235,646 and No. 4,238,248 describe procedures for producing can body stock of various aluminum alloys from strip continuously cast in a casting machine, preferably of the type having a plurality of continuously moving chilling blocks arranged in two sets rotating in opposite senses to form a casting cavity to which the aluminum alloy is supplied for solidification in contact with the blocks. In these procedures, the cast strip is subjected to a holding period at elevated temperature before hot rolling. The patents further describe the cast strip as having a cell size or dendritic arm spacing preferablay of about 5-15 microns in the region of the strip surface and preferably of about 50-80 microns in the center of the strip thickness. After the holding period, the strip is initially reduced by hot rolling under conditions such that the temperature of the strip at the end of the hot rolling step is at least 280.degree. C., and is then further reduced to can stock gauge by cold rolling.